Method for Assessing Source Zone Natural Depletion at Chlorinated Aliphatic Spill Sites

This work focuses on the site-specific assessment of source zone natural attenuation (SZNA) at chlorinated aliphatic hydrocarbon (CAH)-impacted sites. The approach is similar in some ways, but different in other ways from recently proposed SZNA assessment paradigms for petroleum-impacted sites. The similarities lie in the organization of the approach around determining: (1) whether or not SZNA is occurring, (2) the current SZNA rate, and (3) what is the future projection for SZNA rate changes and the final state of the source zone. Differences lie in how those rates are determined, especially with respect to the quantities measured and data reduction. Petroleum-impacted site SZNA approaches emphasize quantifying fluxes of electron acceptors, while the proposed CAH assessment approach emphasizes quantifying parent and daughter compound fluxes. A paradigm for assessing SZNA at CAH sites is presented and its use is illustrated, for example former dry cleaner site, where the SZNA rate was approximately 3.5kg/year as tetrachloroethylene (PCE) with about 80% of the mass loss attributed to groundwater flow and 20% attributed to vapor transport.